Infrared cameras with an infrared image sensor often use two wavelength ranges of a 3 to 5 μm band and an 8 to 12 μm band each being a wavelength band, called the atmospheric window, where absorption due to water molecules and so on in the air is small. Herein, in particular, since the 8 to 12 μm band corresponds to the peak radiation wavelength of blackbody radiation near room temperature, this band is widely used for measuring the distribution of temperatures such as temperatures of the human body. In an infrared device such as an infrared camera, a material having a high infrared transmittance in the above-mentioned band is used as a window member adapted to prevent entry of dust or the like from the outside and to sufficiently introduce infrared light. For example, chalcogenide-based compounds typified by zinc selenide (ZnSe) and zinc sulfide (ZnS), germanium (Ge), and so on are widely used.
As a window member for infrared light with a longer wavelength, KRS-5 as a mixed crystal of thallium iodide (TlI) and thallium bromide (TlBr) and KRS-6 as a mixed crystal of thallium chloride (TlCl) and thallium bromide (TlBr) are well known. KRS-5 has a high transmittance for infrared light of up to 40 μm while KRS-6 has a high transmittance for infrared light of up to 20 μm. However, even with such a material, there is a problem that a surface is roughened due to deliquescence to cause a change in optical properties in a severe environment such as an aircraft system.
A lens member or a window member of an infrared device is required to demonstrate its performance even in a relatively severe environment and thus should be formed with an infrared-transmitting film for the purpose of protecting a surface thereof. There are many proposals for such an infrared-transmitting film.
For example, there are an infrared-transmitting film obtained by stacking a two-layer film of a PbF2 film and a ThF4 film on a ZnSe substrate (Patent Document 1), an infrared-transmitting film obtained by stacking a three-layer film of YF3, ZnS, and YF3 on a ZnSe or ZnS substrate (Patent Document 2), an infrared-transmitting film obtained by forming respective films of ZnS, Ge, ZnS, BaF2, and ZnS on a Ge substrate (Patent Document 3), an infrared-transmitting film obtained by forming respective films of ZnS or ZnSe, Ge, ZnS, and BaF2 on a Ge substrate (Patent Document 4), and so on.
As proposals referring to the protection of a substrate surface, there are, for example, an optical element comprising a base layer of ZnS or the like and a film comprising a base layer of Y2O3 or the like with an elastic modulus twice or more that of the base layer of ZnS or the like and being transparent also in the visible light region (Patent Document 5), an infrared-transmitting film using ZnSe or ZnS as a substrate and comprising an adhesion reinforcing layer of TiO2 or Y2O3 and an abrasion resistance reinforcing layer of Y2O3 as an outermost Layer (Patent Document 6), a structure in which YF3 excellent in environmental resistance such as water resistance or moisture resistance is used as a low refractive index layer for use as an infrared-transmitting film and a structure in which Y2O3 is further disposed as an adhesion reinforcing layer on the YF3 layer to provide a highly durable film (Patent Document 7), a structure in which diamond-like carbon is used as an outermost layer which serves as an abrasion resistance reinforcing layer of an infrared-transmitting film (Patent Document 8), and so on.
In the above-mentioned documents, the feature is to use the zinc sulfide (ZnS) substrate, the zinc selenide (ZnSe) substrate, the germanium (Ge) substrate, or the like. However, ZnS has a disadvantage in that the transmittance for infrared light is low while ZnSe and Ge have a disadvantage in that the price is high. Therefore, a material which is high in infrared transmittance and low in price is required for a window member that is required to efficiently introduce infrared light.
On the other hand, for example, a metal halide such as barium fluoride (BaF2) or sodium chloride (NaCl) is high in infrared transmittance and low in price and thus is a material suitable as a window member. In particular, the barium fluoride (BaF2) has a high transmittance up to a wavelength of about 13 μm and its corrosion effect on water is small unless it reaches 500° C. or higher (Patent Document 9).
On the other hand, BaF2 is a brittle material. When a comparison is made in terms of Vickers hardness, BaF2 shows a small value of 88 to 94 kgf/mm2 while Ge shows 1000 to 1500 kgf/mm2, ZnS shows about 230 kgf/mm2, and ZnSe shows about 110 kgf/mm2. Therefore, in order to use a BaF2 substrate as a window member or a lens member of an infrared device, it should be provided with a protective layer for ensuring the mechanical strength (Patent Document 9).